This invention relates to fuel cell systems, and more particularly to water separators therefor.
Fuel cells in general, and PEM fuel cells in particular, have been proposed for use as electrical power plants to replace internal combustion engines, among other applications. PEM fuel cells are well known in the art, and include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The solid polymer electrolyte is typically made from an ion-exchange resin such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin. The MEA is sandwiched between a pair of electrically conductive elements which serve both as current collectors and means for distributing the fuel cell""s gaseous reactants over the surfaces of the electrodes. In such PEM fuel cells, hydrogen is the anode reactant (i.e. fuel), oxygen (i.e. from air) is the cathode reactant (i.e. oxidant), and they react together to produce electricity and water. The cathode/air stream (and sometimes the anode/H2 stream) is typically humidified to keep the ion-exchange membrane from drying out.
Some fuel cell systems use pressurized, or liquid, hydrogen fuel to fuel the fuel cell. Others store the hydrogen chemically as a thermally dissociable hydride, or physiochemically by heat-releasable adsorption on a suitable adsorbent (e.g. carbon nanofibers). For vehicular applications however, it is desirable to dissociate hydrogenous liquids such as gasoline, methanol, or the like to provide the hydrogen used by the fuel cell owing to the ease with which they can be stored on the vehicle. Gasoline is particularly desirable owing to the existence of a nationwide supply infrastructure therefor. To release their hydrogen, hydrogenous liquids are dissociated in a so-called xe2x80x9cfuel processorxe2x80x9d.
One known fuel processor for dissociating gasoline to form hydrogen is a two stage chemical reactor often referred to as an xe2x80x9cautothermal reformerxe2x80x9d. In an autothermal reformer, gasoline and water vapor (i.e. steam) are mixed with air and pass sequentially through two reaction sections, i.e. a first xe2x80x9cpartial oxidationxe2x80x9d (POX) section, and a second xe2x80x9csteam reformingxe2x80x9d (SR) section. In the POX section, the gasoline reacts exothermically with a substoichiometric amount of air to produce carbon monoxide, hydrogen and lower hydrocarbons (e.g. methane). The hot POX reaction products pass into the SR section where the lower hydrocarbons react with the steam to produce a reformate gas comprising principally hydrogen, carbon dioxide, carbon monoxide and nitrogen. The SR reaction is endothermic, but obtains its required heat from the heat that is carried forward into the SR section from the POX section by the POX effluent. One such autothermal reformer is described in International Patent Publication Number WO 98/08771 published Mar. 5, 1998. The process of producing hydrogen from methanol is similar to that used for gasoline except that the POX step is eliminated and the methanol is delivered directly to a steam reformer where it reacts with steam to produce a reformate comprising H2, CO2 and CO. One known fuel processor for dissociating methanol is a steam reformer such as described in U.S. Pat. No. 4,650,727 to Vanderborgh.
The carbon monoxide concentration in the reformate exiting a steam reformer is too high for the reformate to be used in a fuel cell without poisoning it. Accordingly, the CO concentration must be reduced to a very low level that is non-toxic to the fuel cell. It is known to cleanse the reformate of CO by subjecting it to a so-called xe2x80x9cwater-gas-shiftxe2x80x9d (WGS) reaction which takes place in a WGS reactor located downstream of the SR reactor. In the WGS reaction, water (i.e. steam) reacts endothermically with the carbon monoxide according to the following ideal shift reaction:
CO+H2Oxe2x86x92CO2+H2
Some CO survives the water-gas-shift reaction, and must be further reduced (i.e. to below about 20 ppm) before the reformate can be sent to the fuel cell. It is known to further reduce the CO content of H2-rich reformate by selectively reacting it with oxygen (i.e. as air) in a so-called PrOx (i.e. preferential oxidation) reaction which is carried out in a catalytic PrOx reactor located downstream of the water-gas-shift reactor. The PrOx reaction is exothermic and proceeds as follows:
CO+1/2O2xe2x86x92CO2
The combination of a WGS reaction followed by a PrOx reaction is usually sufficient to cleanse the reformate enough that it can be then used in the fuel cell.
It is known to burn the cathode and anode tailgases exiting the fuel cell in a combuster to remove any hydrogen from the system""s exhaust gasses, and to provide heat for use elsewhere in the system. e.g. (1) to heat a methanol reformer, or (2) to vaporize liquid fuel and water for use in the system. Moreover, it is known that water management of fuel cell systems that are to be used for vehicular applications (i.e. cars, trucks, buses etc.) is an important consideration. In this regard, it is desirable to collect the water generated by the fuel cell and reuse it elsewhere in the system (e.g., in a fuel processor, water-gas shift reactor or humidifier) where it is needed rather than storing an extra supply of water on-board for such system needs. Moreover, it is desirable to minimize the amount of liquid water in the various system streams so as not to detrimentally effect reactors supplied by such streams. Hence for example, liquid water should be eliminated from the fuel cell tailgases, and particularly the cathode tailgas, that are supplied to the combuster so as not to drown the combuster catalyst, or otherwise suppress combustion of the tailgases therein. Similarly, it is desirable to insure that the H2-rich fuel gas supplied to the anode and/or cathode sides of the fuel cell contain little or no liquid water that could either drown the catalyst or flood the fuel cell and thereby reduce its effectiveness. It is likewise desirable to recapture water from the exhaust system from the system""s combustor. Accordingly, it is known to provide one or more mechanical water separators at various locations within the system to remove liquid water from the various gas streams and direct it to a water collection site. This practice adds additional equipment to the system which is undesirable for vehicular applications particularly since the water separators that have been used heretofore have been large, inefficient and/or have too much pressure drop which is wasteful of system energy.
The present invention mitigates the undesirable impact of mechanical water separators in fuel cell systems for vehicular applications by providing a separator which (1) is compact so as not to consume much space in the vehicle""s engine compartment, (2) has a high separating efficiency so as to ensure a high degree of water removal and collection, and (3) has a low pressure drop.
The present invention relates to a fuel cell system having a compact, efficient, low-pressure-drop water separator for removing liquid water droplets from water-laden system streams. The invention is applicable to all fuel cell systems that comprise a fuel cell, a source of H2-rich fuel-gas for fueling the fuel cell, a source of oxygen (e.g. air) for electrochemically reacting with the H2 in the fuel cell, and a reservoir for collecting water separated from the various system streams for reuse elsewhere in the system (e.g. in a fuel processor or humidifier for the H2 and/or O2 streams). Broadly speaking, the invention is applicable to a fuel cell system comprising (1) a fuel cell, (2) a source of H2-rich fuel-gas providing a fuel stream for said fuel cell, (3) a source of oxygen providing an oxidant stream for electrochemically reacting with the H2-rich fuel-gas in the fuel cell, (4) an anode exhaust stream comprising H2-depleted fuel-gas exiting the fuel cell, (5) a cathode exhaust stream comprising water, oxygen and nitrogen exiting the fuel cell, (6) at least one water separator for removing water from at least one of the system""s streams that is laden with liquid water, and (7) a reservoir for collecting the water removed from the water-laden stream for reuse within the system. More specifically, the invention is directed to such a fuel cell system where the separator is a cyclonic separator that has a collection tube having an internal cylindrical wall defining a collection chamber through which the water-laden stream flows. The collection tube has an inlet through which the water-laden stream enters the chamber, and an outlet through which the separated water exits the chamber. A swirler at the inlet to the collection tube imparts a whirling motion to the water-laden stream entering the chamber that centrifugally propels the water out of the water-laden stream onto the wall while urging the water along the wall toward the outlet. According to one embodiment of the invention, the swirler comprises a plurality of arcuate vanes positioned in the inlet to the collection tube. Alternatively, the swirler may comprise the inlet to the collection tube being arranged and adapted so as to introduce the water-laden stream tangentially into the collection tube. A sump underlies the outlet of the collection tube to collect the water that has migrated to the outlet along the wall. A baffle between the outlet and the sump serves to admit the water into the sump while preventing water in the sump from escaping the sump and re-entraining in the stream passing through the separator. A drain communicates with the sump to drain-off the water from the sump into the reservoir. A valve that is operatively associated with the drain controls draining of the water from the sump so as to maintain a sufficient level of water in the sump to provide a water seal that prevents the stream passing through the separator from escaping the separator via the drain. A liquid level switch is coupled with the sump to trigger closing of the valve before that level drops too low to maintain the water seal. The separator has an exhaust tube for exhausting the water-depleted vapor stream from the separator. The exhaust tube has (1) a mouth at one end that is substantially concentric with the collection tube radially inboard the cylindrical inner wall of the collection tube to receive the water-depleted stream passing through the separator from the collection chamber, and (2) an outlet end for discharging the water-depleted vapor stream from the separator.
According to a preferred embodiment, the invention involves a fuel cell system comprising (1) a fuel cell, (2) a fuel processor for converting a hydrogenous fuel such as methanol or hydrocarbon into a CO-containing, H2-rich fuel-gas for fueling the fuel cell, (3) a water-gas-shift reactor downstream of the fuel processor for reacting the CO-containing, H2-rich fuel-gas with steam to increase its H2 content and decrease its CO content, (4) a water separator downstream of the fuel cell to remove water from a water-laden effluent therefrom (e.g. cathode tailgas), and (5) a reservoir for collecting the water removed from the effluent for reuse in the fuel processor and/or the water-gas-shift reactor. More specifically, the preferred fuel cell system has a cyclonic, water-gas separator that mechanically separates water from the effluent without loss of any of the gas that carries the water. The separator comprises a collection tube that has an internal cylindrical wall that defines a collection chamber through which the water-laden stream flows and is separated (i.e. into water and carrier gas). The collection tube has an inlet through which the water-laden gas enters the chamber, and an outlet through which water exits the chamber. A swirler located at the inlet imparts a whirling motion to the gas that (1) centrifugally propels the water onto the cylindrical wall, and (2) urges the water layer that forms on the wall toward the outlet. A sump underlies the outlet of the collection tube to collect the water that has been pushed to the outlet along the wall by the swirling gas. A baffle between the outlet and the sump serves to admit the water into the sump while preventing water in the sump from escaping the sump and reentering the carrier gas. A preferred such baffle has a plurality of apertures therein through which the water enters the sump, and will, most preferably, surround the outlet of the collection tube. The sump has a drain to .drain away any water that accumulates in the sump. The drain includes a shut-off valve that controls on-off flow through the drain, the valve closes to prevent outflow of water from the sump while there is still sufficient water in the sump to provide a liquid seal that prevents escape of the carrier gas through the drain. Opening and closing of the valve is controlled by a switch (e.g. a float-switch) that determines the level of the water in the sump, and triggers closing of the valve before the level of the water in the sump gets too low to prevent the escape of the carrier gas through the drain. The separator also includes an exhaust tube having a mouth at one end that is substantially concentric with, and radially inboard the wall of, the collection tube. The mouth of the exhaust tube receives water-depleted carrier-gas from the longitudinal central region of the chamber. An exhaust end at the other end of the exhaust tube opposite the mouth discharges the water-depleted carrier-gas from the separator, e.g. into a combuster. In a most preferred embodiment of the invention, the fuel cell is a PEM fuel cell and the system further includes a humidifier upstream of the fuel cell that receives water from the reservoir for humidifying the cathode air stream. The water could likewise be directed from the reservoir to the fuel processor or water-gas-shift reactor.
In one embodiment of the separator that is adapted for in-line installation in a fuel cell system, the collection and exhaust tubes are aligned with each other along a common axis such that the inlet to the collection tube confronts the mouth of the exhaust tube and is aligned with the exhaust end of the exhaust tube. In another embodiment, the mouth of the exhaust tube lies on the same longitudinal axis as the collection tube, but the exhaust end of the exhaust tube lies along a different axis that is at an acute angle (e.g. 90xc2x0) to the longitudinal axis of the collection tube. In this latter embodiment, the collection tube has an inlet at one end, an outlet at the opposite end, an endwall adjacent the outlet, and the mouth of the exhaust tube confronts the endwall such that the gas flows (a) in a first general direction along the wall of the collection tube from the inlet toward the endwall, and (b) is then deflected off the end wall so as to flow through the center of the collection tube in a second general direction opposite the first general direction and into the mouth of the exhaust tube. This latter embodiment is particularly useful for non-in-line installations.